Conventionally, in the image forming apparatus according to Patent Document 1 for example, in order to prevent change in image density due to change with time of a developer (hereafter called “toner”), changes of the development characteristics due to the environment, etc., density detection patterns of low duty density, medium duty density, and high duty density are printed on a transfer belt for carrying the print medium. Then, by detecting the density detection patterns by a density detection part and correcting the density values of these detected density detection patterns, the image density is appropriately adjusted.